1. Field of the Invention
The present invention relates to a sensor unit with a piezoelectric element for three-dimensionally sensing the magnitude and the direction of a physical quantity applied from the outside.
2. Description of Related Art
In the automobile, mechanical, and related industries, there is an increasing need for a sensor capable of accurately detecting a physical quantity such as force, acceleration, magnetic field, or the like. In particular, there is a need for developing a small-sized sensor capable of detecting such a physical quantity for each of two- or three-dimensional components.
One known technique to realize such a sensor is to dispose a plurality of piezoelectric sensor elements on a flexible plate provided with an operating member (as disclosed in Japanese Patent Application Laid-open No. 5-26744).
In this sensor, the flexible plate is deformed in response to a physical quantity applied from the outside to the operating member, and the piezoelectric element generates a charge corresponding to the deformation of the flexible plate thereby three-dimensionally detecting the magnitude and the direction of the physical quantity using the single sensor unit (hereinafter, this type of sensor is referred to as a "three-axis sensor").
As an example of a three-axis sensor, an acceleration sensor of the type using a weight as an operating member is described below with reference to FIG. 2. As shown in FIG. 2, when an acceleration of a is applied from the outside on the sensor, a weight 1 experiences an inertial force f in a direction opposite to the acceleration a. As a result, a flexible plate 3, which extends in a horizontal direction between the weight 1 and a supporting base 2, has a deformation corresponding to the inertial force f.
Depending on the direction and the amount of the deformation 4, corresponding amount of charges are generated in respective piezoelectric elements 5 disposed on the flexible plate 3. By detecting these charges, it is possible to perform a three-dimensional detection of the acceleration applied from the outside.
This type of sensor is described in further detail below with reference to FIGS. 3(a) and 3(b). In the sensor shown in FIGS. 3(a) and 3(b), the center of the bottom plane of a cylindrical weight 10 to which a flexible plate 12 is attached is defined as the origin O. The plane extending in parallel to the flexible plate 12 passing through the origin O is defined as an X-Y plane, and X and Y axes are defined in this X-Y plane so that X and Y axes are perpendicular to each other. Furthermore, a Z axis is defined so that it extends in a direction perpendicular to the X-Y plane and passes through the origin O.
In this structure, each portion of the piezoelectric material located between one pair of upper and lower electrodes is referred to as a "piezoelectric sensor element". In the specific example of the sensor shown in FIGS. 3(a) and 3(b), four piezoelectric sensor elements each consisting of a particular portion of the piezoelectric material and a pair of electrodes are disposed in the X and Y directions on the flexible plate 12, and additional eight piezoelectric sensor elements for use in detection in the Z direction are disposed.
In this sensor, the respective components of the inertial force f applied on the weight 1 by the external acceleration a are determined by the amounts of charge generated in the respective piezoelectric sensor elements as described below. That is, the X-axis component f.sub.X of the inertial force is detected by the piezoelectric sensor elements E.sub.1 -E.sub.4 as shown in FIG. 4(a). Similarly, the Y-axis component f.sub.Y of the inertial force is detected by the piezoelectric sensor elements E.sub.5 -E.sub.8 (not shown). On the other hand, the Z-axis component f.sub.Z, shown in FIG. 4(b), of the inertial force is detected by the piezoelectric sensor elements E.sub.9 -E.sub.12 and also by the piezoelectric sensor elements E.sub.13 -E.sub.16.
The directions of the respective components are determined on the basis of the charge polarity pattern. For instance, as shown in FIG. 4(a), a charge polarity pattern of "+-+-" appears on the upper surface of the piezoelectric material, while a charge polarity pattern of "+--+" appears on the upper surface of the piezoelectric material in FIG. 4(b), wherein the pattern is seen from left to right in both cases.
From the resultant force of the combination of the detected components f.sub.X, f.sub.Y, and f.sub.Z, the direction and the magnitude of the inertial force f (and therefore the magnitude and direction of the external acceleration a) can be determined in a three dimensional fashion using the single small-sized sensor.
In order for the three-axis sensor to accurately detect the inertial force f, it is required to accurately detect each component f.sub.X, f.sub.Y, and f.sub.Z in the X, Y, and Z directions, respectively.
However, the detection of one component can be influenced by the presence of other components because the three-axis sensor described above detects each component in the X, Y, and Z directions by means of the single flexible plate having a weight and extending in a horizontal direction.
That is, the detection limit, which is determined by the spring constant of the flexible plate, is further reduced if the single flexible plate is used for detection in both directions. This results in a reduction in the detection range of the sensor.
Although the above problem may be avoided to a certain extent by increasing the hardness or the spring constant of the flexible plate, the result is a reduction in the sensitivity.